1. Field of the Invention
The present invention relates to a color filter substrate usable for a display device of, for example, TVs, personal computers, wordprocessors and office automation apparatuses, and a method for producing the same.
2. Description of the Related Art
Color filter substrates including a transparent base plate and a color filter layer are conventionally used in flat panel display devices such as liquid crystal display devices.
As methods for producing a color filter substrate, dyeing, electrodeposition, pigment dispersion and the like are generally known. Among these methods, pigment dispersion is in wide use because dyeing and electrodeposition cause the colors to be easily faded and also result in unsatisfactory purity of color.
By the pigment dispersion, photosensitive resin films containing pigments dispersed therein are applied on a base plate and patterned. An exemplary formation process of a color filter substrate by pigment dispersion will be described with reference to FIG. 11.
First, a base plate is washed (block 11a). Next, as shown in block 11b, a two-film chrome layer is formed by sputtering, and a resist is applied thereon. Then, exposure, development and wet etching are sequentially performed, and then the resist is removed. Thus, a layer acting as a black matrix is formed.
As shown in block 11c, a photosensitive resin containing a red pigment dispersed therein is spin-coated on the base plate so as to cover the black matrix, and then pre-baked. Then, the photosensitive resin is treated by exposure, development and pre-baking, thereby forming a red color layer.
As shown in block 11d, a photosensitive resin containing a green pigment dispersed therein is spin-coated on the base plate so as to cover the black matrix and the red color layer, and then pre-baked. Then, the photosensitive resin is treated by exposure, development and pre-baking, thereby forming a green color layer.
As shown in block 11e, a photosensitive resin containing a blue pigment dispersed therein is spin-coated on the base plate so as to cover the black matrix and the red and green color layers, and then pre-baked. Then, the photosensitive resin is treated by exposure, development and pre-baking, thereby forming a blue color layer.
The resultant substrate is inspected (block 11f), washed and then baked (11g). Thus, the color filter substrate is obtained.
As a method for producing a color filter substrate including a color filter containing pigments, a dry film laminate (DFL) method using a dry film is shown in FIG. 12.
First, a base plate is washed (block 12a). Next, as shown in block 12b, a dry film containing a red pigment dispersed therein is laminated on the base plate, and a protection film of the dry film is removed. Then, the dry film is treated by exposure, removal of a cushioning layer and development of a color layer through a photomask, and pre-baking. Thus, a red color layer is formed.
Next, as shown in block 12c, a dry film containing a green pigment dispersed therein is laminated on the base plate so as to cover the red color layer, and a protection film of the dry film is removed. Then, the dry film is treated by exposure, removal of a cushioning layer and development of a color layer through a photomask, and pre-baking. Thus, a green color layer is formed.
Next, as shown in block 12d, a dry film containing a blue pigment dispersed therein is laminated on the base plate so as to cover the red and green color layers, and a protection film of the dry film is removed. Then, the dry film is treated by exposure, removal of a cushioning layer and development of a color layer through a photomask, and pre-baking. Thus, a blue color layer is formed.
Next, as shown in block 12e, a dry film containing a black pigment dispersed therein is laminated on the base plate so as to cover the red, green and blue color layers, and a protection film of the dry film is removed. Then, the dry film is treated by exposure, removal of a cushioning layer and development of a color layer through a photomask, and pre-baking. Thus, a layer acting as a black matrix is formed.
The resultant substrate is inspected (block 12f), washed and then baked (12g). Thus, the color filter substrate is obtained.
The DFL method has the following advantages over the spin-coating described above with reference to FIG. 11 :
(1) A uniform desired layer thickness is obtained more easily by merely using a dry film having a desired layer thickness. Since the dry film contains almost no moisture and the color filer is formed in the state of being relatively easily polymerizable, it is not necessary to consider a reduction in the thickness after baking, as is necessary in the case of spin-coating. PA1 (2) Whereas, in the case of spin-coating, only a small part of the material is left on the base plate to be used and the rest is discarded at the time of application, the DFL method allows a significant part of the material to be used. Thus, the material is not wasted. PA1 (3) Since application of a dry film is easier than spin-coating, the production cost is lower.
A color filter substrate obtained in such a manner is generally usable as a counter substrate, which is combined with an active matrix substrate to produce a liquid crystal display device as described in, for example, Japanese Laid-Open Publication No. 58-172685. Alternatively, a color filter layer of the color filter substrate can be provided in an active matrix substrate as described in, for example, Japanese Laid-Open Publication No. 6-242433. In other words, such an active matrix substrate including a color filter layer is obtained by providing an interlayer insulative layer in the active matrix substrate of Japanese Laid-Open Publication No. 58-172685 with the function of a color filter layer. An exemplary active matrix liquid substrate including the color filter layer will be described with reference to FIGS. 2 and 3A. FIG. 2 is a plan view of one picture element area of an active matrix substrate 700. FIG. 3A is a cross-sectional view of the active matrix substrate 700 taken along line A-A' of FIG. 2.
As shown in FIG. 2, an active matrix substrate 700 includes a plurality of gate signal lines 25 acting as scanning lines and a plurality of signal lines 24 acting as signal lines perpendicular to the gate signal lines 25. A plurality of picture element electrodes 10 (only one is shown in FIG. 2) formed of a transparent conductive material are provided in a matrix in the state of being surrounded by the gate signal lines 25 and the source signal lines 24. A plurality of thin film transistors (TFTs) 21 (only one is shown in FIG. 2) are provided at intersections of the gate signal lines 25 and the source signal lines 24.
With reference to FIG. 3A, the cross-sectional structure of the active matrix substrate 700 will be described. For simplicity, the description will be directed to one picture element area. The active matrix substrate 700 includes a transparent insulative base plate 5 (referred to simply as the "base plate"), a gate electrode 12 provided on the base plate 5, a gate insulative layer 13 provided on the base plate 5 so as to cover the gate electrode 12, a semiconductor layer 14 provided on the gate insulative layer 13 above the gate electrode 12, and a channel protection layer 15 provided on a central part of the semiconductor layer 14. The active matrix substrate 700 further includes a source electrode 16a and a drain electrode 16b provided on the gate insulative layer 13 so as to cover the semiconductor layer 14 and the channel protection layer 15. The source electrode 16a and the drain electrode 16b are formed of n.sup.+ -Si and are separated from each other on the channel protection layer 15.
The source electrode 16a is connected to the source signal line 24 including a transparent conductive layer 17a and a metal layer 18a provided on the transparent conductive layer 17a. The drain electrode 16b is connected to a two-film layer including a transparent conductive layer 17b and a metal layer 18b. The gate electrode 12 is branched from the gate signal line 25 (FIG. 2). The gate electrode 12, the source electrode 16a, the drain electrode 16b, the semiconductor layer 14, the channel protection layer 15, and an area of the gate insulative layer 13 below the semiconductor layer 14 are included in the TFT 21. The TFT 21 is driven by a signal inputted to the gate electrode 12 from the gate signal line 25. A voltage from the source signal line 24 is applied to each of the picture element electrode 10 via the source electrode 16a.
Returning to FIG. 3A, the active matrix substrate 700 further includes the interlayer insulative layer 8 provided on the gate insulative layer 13 so as to cover the TFT 21, the transparent conductive layers 17a and 17b, and the metal layers 18a and 18b. The interlayer insulative layer 8 has a contact hole 11 formed therethrough. The picture element electrode 10 is provided on the interlayer insulative layer 8. The transparent conductive layer 17b is extended to below the contact hole 11. An extended part of the transparent conductive layer 17b acts as a connection electrode 17c (FIG. 2). The picture element electrode 10 is electrically connected to the drain electrode 16b through the transparent conductive layer 17b.
Referring to FIG. 2 again, a portion of the connection electrode 17c is extended to a central part of the picture element electrode 10 substantially in parallel to the source signal line 24 and another portion of the connection electrode 17c substantially in parallel to the gate signal line 25 in the central part. In the central part, the connection electrode 17c substantially overlaps a storage capacitance line 22 provided in parallel to the gate signal line 25. The overlapping connection electrode 17c and storage capacitance line 22 act as electrodes forming a storage capacitance.
As shown in FIG. 3A, the interlayer insulative layer 8 includes a color portion in correspondence with the picture element electrode. In the picture element area shown in FIG. 3A, the interlayer insulative layer 8 includes a red color portion 1 (a green color portion 2 and a blue color portion 3 are also partially shown). The interlayer insulative layer 8 also includes a black color portion 4 on the TFT 21 and the vicinity thereof.
Although not shown, the active matrix substrate 700 further includes an alignment layer of polyimide or the like provided on the interlayer insulative layer 8 so as to cover the picture element electrodes 10. The alignment layer is provided at least in a display area. The alignment layer is provided with an aligning function by rubbing, UV radiation or the like.
Again although not shown, a counter substrate includes a transparent insulative base plate, a transparent counter electrode formed of ITO or the like and provided on the base plate, and an alignment layer provided on the counter electrode. The alignment layer is provided at least in the display area.
The active matrix substrate 700 (FIG. 3A) and the counter substrate are combined together to produce a panel of a liquid crystal display device in, for example, the following conventional manner, which will be discussed generally without reference to drawings.
A sealing material is applied on a surface of one of the substrates along the perimeter thereof (i.e., seal area) except for a liquid crystal injection opening. The sealing material is applied by, for example, printing. On a surface of the active matrix substrate, a conductive material is applied to a signal input terminal for the counter electrode and also spacers are scattered. The two substrates are positionally aligned and assembled together. The sealing material is cured by heating. A liquid crystal material is injected into a space between the two substrates through the liquid crystal injection opening. Thus, a liquid crystal layer is formed. The liquid crystal injection opening is then sealed by a sealing agent.
The liquid crystal layer is optically modulated by the potential difference between each of the picture element electrodes 10 and the counter electrode, and such an optical modulation is visually recognized as a display pattern. As a switching element for selectively driving the picture element electrodes, metal-insulator-metal (MIM) devices are also generally used in lieu of the TFTs 21.
In the active matrix substrate 700, the picture element electrodes 10 overlap the gate signal lines 25 and the source signal lines 24. Such a structure has advantages of: (1) the numerical aperture of the resultant liquid crystal display device is increased, and (2) defective alignment of liquid crystal molecules is prevented since the electric field caused by the signal lines is shielded.
A color filter generally includes a red color layer, a green color layer and a blue color layer as well as a black matrix for preventing mixture of the colors and light leakage.
The active matrix substrate 700 shown in FIGS. 2 and 3A does not require a black matrix in the display area, in which the picture element electrodes 10 for display are provided, since the gate signal lines 25 and the source signal lines 24 are present in the gaps between the picture element electrodes 10. Accordingly, only a so-called frame area surrounding the display area needs to be shielded against light in the following manner, which eliminates the step of forming a black matrix.
The frame area includes the gate signal lines 25 and the source signal lines 24. The gaps between each two adjacent gate signal lines 25 are covered by a layer provided on the same level as the source signal lines 24 so as to insulate the two adjacent gate signal lines 25 from each other, and the gaps between each two adjacent source signal lines 24 are covered by a layer provided on the same level as the gate signal lines 25 so as to insulate two adjacent source signal lines 24 from each other. Such layers, which are generally formed of at least a metal material since the gate and source signal lines 25 and 24, respectively, are formed of at least a metal material, are lustrous when seen from the front surface of the active matrix liquid crystal display device. In order to cover the lustrous layers, the color filter layer is extended to the frame area.
In the case where the color filter layer is included in the active matrix substrate, it is not necessary to consider the alignment error occurring when the active matrix substrate and the counter substrate are combined. Thus, the numerical aperture is further increased.
The structure, in which the color filter layer provided either in the counter substrate or the active matrix substrate is extended to the frame area, has a problem in that a portion of the sealing material exudes outside the panel when the sealing material is heated to be cured. More specifically, immediately before the sealing material is completely cured, a solvent and a filler for adjusting the viscosity of the sealing exudes outside the seal area due to the non-uniformity in thermal distribution and non-uniform mixture.
Such a phenomenon is closely related to the distance between the two substrates in the seal area. The distance between the two substrates is controlled to be a desired distance using both spacers contained in the sealing material and spacers scattered on the active matrix substrate. Both heat and pressure are simultaneously applied to the seal area. When the distance between the two substrates in the seal area is excessively small, a high pressure is concentrated on the sealing material in addition to heat. This is a cause of the exudation.
Specifically when the inner surfaces of the two substrates are flat, there is no space for the pressure applied on the sealing material to be released. Accordingly, the above-described phenomenon occurs more easily in such situations, and in the worst case, portions of the sealing material reach the display area. Further, portions of the sealing material (even portions not reaching the display area) remain uncured after the completion of the liquid crystal display device because the exuded components are not in a satisfactory state in which the main curing components are mixed uniformly. Such components may disadvantageously exude into the display area during the use of the liquid crystal display device. Such a phenomenon may result in a defective display or other problems which reduce overall reliability.
One way in which to reduce the problems discussed above with respect to exuding sealing material is to provide a non-planar face (as opposed to a flat face) on which the sealing material is applied. For example, a specific color layer can be formed thinner than the other color layers in the entire area in which the color layers are formed. In an alternative example, all the color layers are formed to have an equal thickness in the display area in order to prevent the chromaticity characteristics from being spoiled, and a specific color layer is formed to be thinner in another area, for example, only the seal area.
Such a method for producing a color filter layer has the following problems.
A dry film is applied on the base plate with a pressure so as to cover another dry film which is already applied. The another dry film, which is a color layer, generally has a thickness of 1 .mu.m to 2 .mu.m. Accordingly, it may be difficult to apply the dry film without forming a space between the dry film which is being applied and the base plate when the dry film is not sufficiently viscous. Air bubbles caused by the space may result in defective patterning along the border between the two color layers, and the air bubbles may cause the dry film to be delaminated in a later stage of production, resulting in contamination of the process.
For example, formation of a black color layer acting as a black matrix will be described with reference to FIGS. 13A, 13B and 13C.
In the structure in which the picture element electrodes overlap the gate signal lines and the source signal lines, a light-blocking layer is not required as described above. However, it is preferable to provide a light-blocking device at least on TFTs in order to prevent an increase of an off-state current in the TFTs caused by the semiconductor layer being irradiated with light incident on the panel.
FIG. 13A is a partial plan view of a color filter layer including a red color portion 1, a green color portion 2 and a blue color portion 3. Each portion has a recess 701 in positional correspondence with a TFT. When a dry film is applied so as to cover the portions 1, 2 and 3 for forming a black color layer in the direction of arrow X10, spaces are formed in the recesses 701 between the base plate and the dry film for the black color layer. Air bubbles are generated in such spaces and thus cause the dry film for the black color layer to be delaminated.
The red, green and blue color portions 1, 2 and 3 are provided in positional correspondence with picture element electrodes in accordance with the shapes of the electrodes. The color portions 1, 2 and 3 can be provided with a recess 702 as shown in FIG. 13B in accordance with the shape and arrangement of the picture element electrodes. When a dry film for forming a black color layer is applied so as to cover the color portions 1, 2 and 3 in the direction of arrow X11, a space is formed in the recess 702 between the base plate and the dry film for the black color layer. Air bubbles are generated in such a space and thus cause the dry film for the black color layer to be delaminated.
FIG. 13C shows a delta arrangement of color portions 1, 2 and 3 generally used in, for example, a monitor for an audiovisual apparatus. Formation of the red color portions 1 and the green color portions 2 leave the areas for blue color portions as gaps 703. When a dry film for forming blue color portions is applied so as to cover the red and green color portions 1 and 2, spaces are formed in the gaps 703 between the base plate and the dry film for the blue color portions. Air bubbles are generated in such spaces and thus cause the dry film for the blue color portions to be delaminated.